1. Field of the Invention
The present invention relates to a television tuner input circuit, and more particularly to an input circuit of a television tuner capable of switching so as to be tunable to plural frequency bands.
2. Description of the Prior Art
An input circuit of a conventional television tuner will be described with reference to FIG. 3. In FIG. 3, the input circuit of the television tuner comprises an input part 61, a CB trap circuit 62, an IF trap circuit 63, a tuning circuit 64, and an output part 65, which are connected in series, and first and second band switching terminals 66 and 67 for switching the tuning circuit 64 to a low band or high band television signal reception state. To the first band switching terminal 66, a switching voltage for switching the tuning circuit 64 to a low band television signal reception state is applied, and to the second band switching terminal 67, a switching voltage for switching the tuning circuit 64 to a high band television signal reception state is applied.
The input part 61 comprises a television signal input terminal 68, a coil 69 connected between the input terminal 68 and the ground, and a DC blocking capacitor 70 connected at one end thereof to the input terminal 68. The input terminal 68, connected with a television antenna (not shown), serves as an input end for television signals. The input circuit 61 is primarily used as provision against lighting.
The CB trap circuit 62 has a coil 71 and a capacitor 72 connected in series between the other end of the capacitor 70 and the ground. A resonance frequency of a series resonance circuit comprising the coil 71 and the capacitor 72 is set to a frequency used with citizen band transceivers, e.g., 27 MHz.
The IF trap circuit 63 comprises a coil 73 and a capacitor 74 the respective one end of which is connected in parallel to the other end of the capacitor 70. A resonance frequency of a parallel resonance circuit comprising the coil 73 and the capacitor 74 is set to an intermediate frequency region of television signals (59 MHz in Japan and 46 MHz in the United States).
The tuning circuit 64, as shown in FIG. 3, comprises a tuning coil 80 for high band reception, a DC blocking capacitor 79, a tuning coil 78 for low band reception, a tuning coil 75 for low band reception, a tuning coil 76 for high band reception, and a DC blocking capacitor 77, which are connected in series, connected in parallel with a varactor diode 81 and a DC blocking capacitor 82, which are connected in series. The varactor diode 81 has an anode connected with the coil 80 and a cathode connected with the capacitor 82. The other ends of the DC blocking capacitor 77 and the EC blocking capacitor 82 are grounded. A connection point between the coil 76 and the capacitor 77 is grounded through a resistor 83 and connected with the switching terminal 66 through a resistor 84. A connection point between the coils 75 and 76 is connected with the switching terminal 67 through a DC stopping capacitor 85 and a resistor 86. The switching terminal 67 is grounded through a resistor 87.
Next, a switch diode 88 is connected between a connection point between the capacitor 85 and the resistor 86 and a connection point between the coil 78 and the capacitor 79. The switch diode 88 has an anode connected with the capacitor 85 and a cathode connected with the coil 78. A connection point between the capacitor 79 and the coil 80 is grounded through a resistor 89, and the cathode of the varactor diode 81 is connected with a tuning voltage supplying terminal 91 through a feeding resistor 90.
The output part 65 comprises a varactor diode 92 having an anode connected with the anode of the varactor diode 81, a DC blocking capacitor 93 connected at one end thereof with the cathode of the varactor diode 92, and a feeding resistor 94 connected between the cathode of the varactor diode 92 and the tuning voltage supplying terminal 91. The other end of the DC blocking capacitor 93 is connected with an amplifier 95 of the next stage so that a television signal outputted by the input circuit of the television tuner is amplified by the amplifier 95.
In the configuration as described above, when the tuning circuit 64 is switched so as to receive television signals of high band, a voltage of, e.g., 5V is applied to the switching terminal 67 for high band reception, and no voltage is applied to the switching terminal 66 for low band reception. At this time, a forward voltage is applied to the switch diode 88, which is brought into conduction, and the tuning circuit 64 operates as a tuning circuit comprising the coils 76 and 80, and the varactor diode 81. As a result, reception of signals of high frequency band is enabled, a signal of a desired tuning frequency is obtained by adjusting voltages applied to the tuning voltage supplying terminal 91, and the signal is outputted to the amplifier 95.
On the other hand, when the tuning circuit 64 is switched so as to receive television signals of low band, a voltage of, e.g., 5V is applied to the switching terminal 66 for low band reception, and no voltage is applied to the switching terminal 67 for high band reception. At this time, a reverse voltage is applied to the switch diode 88, which is brought out of conduction, and the tuning circuit 64 operates as a tuning circuit comprising the coils 75, 76, 78, and 80, and the varactor diode 81. As a result, reception of signals of low frequency band is enabled, a signal of a desired tuning frequency is obtained by adjusting voltages applied to the tuning voltage supplying terminal 91, and the signal is outputted to the amplifier 93.
However, in the television tuner input circuit, the CB trap circuit 62 and the IF trap circuit 63 have the property of transmitting signals at a frequency band (86 to 109 MHz), which is half the frequency band of high band television signals (171 to 217 MHz in Japan and 175 to 211 MHz in the United States). For this reason, in high band reception, signals having half of signal frequencies to be received are not so highly attenuated because they are attenuated due to the tuning properties of the tuning circuit 64 only.
The dotted line C of FIG. 2 shows the selection property of the television tuner input circuit. As shown in the drawing, signals at a frequency band (86 to 109 MHz), whose frequencies are is half of those of the frequency band of high band television signals, are not sufficiently attenuated.
In such a state, signals (FM broadcasting signals) at an FM broadcasting band, whose frequencies are half of those of high band television signals, are inputted to the amplifier 95 to amplify the signals, while harmonics having twice the frequencies, that is, the harmonics of the same frequencies as signals of frequencies to be received are generated and interfere with television signals to be received.